At my Fine Print Photographer's Workshop I often get requests to process film and make prints from photographers who have uprated their film speed. Occasionally this is fully justified: in situations such as available light interiors, where no picture would have been possible without a film speed increase, or to achieve a deliberately stark effect. Usually though the photographer has uprated for no apparent reason, as though there were some points to be won by doing so. It's almost macho to uprate. They even do it when it makes matters more difficult in taking - when there is glaring sun and an EI 1600 film speed leaves the camera without suitable shutter speed and aperture combinations. I am sure film manufacturers are very aware of the sales appeal of high film speeds. There are few extra sales in claiming that your film is slower than everybody else's, but probably quite a few if it is claimed to be high speed and ultra fine grain as well. To be fair, some major steps have been made in the last decade thanks to new grain technology. Some ISO 400 films do now have the grain of the old 125 emulsions, and new 100 emulsions better the grain of older ultra-slow types. What is more to the point is whether any of those figures are valid in practice anyway. I, and many other more expert photographers, have found that makers' speed ratings don't stand up in practice, and far from being uprated, they should be downrated. Typically, standard ISO 400 emulsions work out at about 160, and 125 at 50 or 64 (I exclude T-Max from this). Sceptical readers are probably right now saying, "Oh yeah" So why do I say this? I mentioned earlier that exposure meters are set to read any scene as if it were 'average' or 'middle' grey: the standard 18% reflectance which manufacturers like Kodak reproduce in their commonly available grey card. I have a question. Who says that the average of a typical scene is 18% grey? Nobody I have asked has been able to tell me how this assumption arose. If it is incorrect, then we immediately have an error effectively in the nominal film speed. I believe the correct 'middle grey' should, in fact, be more like 9%. There is much confusion about this 'average' or 'middle' terminology as well. When asked, many of my clients say they visualise it as the exact mid-point between black and white; the reading given by a meter pointed at, say, a chessboard with an exactly equal number of equally sized black and white squares. In fact, such a scene would give a reading, theoretically, of exactly half the reflected light of a totally white board. Half the light means the same as one stop less or one zone less; so, if pure white is zone X, then the chess board would indicate zone 1X on the meter, not the stipulated zone V for middle grey. To get a zone V reading, the chessboard would have to have around 80% of its area black. Try it with a mock up and you will see this is so. But is this 18% right anyway? If the luminance range of a typical photographic scene is around seven stops - and a surprisingly large proportion do seem to be - then that is 2.1 in log terms. Half of this is 1.05, which translates to a reflectance value of around 9%. (If you're not into sensitometry, take my word for it!) Nevertheless exposure meters are supposedly set to render anything they read as standard 18% grey, which is zone V in the zone system. Since zone 0 is solid black, then we can work out what should happen at each zone if the maker's speed is as claimed. By then running a very simple test, we can check if the claim is right on our own equipment (in a way that will eliminate all questions about what is real middle grey) and if it isn't what the correct speed is for us. Makers set film speeds in a laboratory using laboratory instruments. We work in real life with ordinary instruments, cameras, enlargers, etc. For instance, have you ever checked your meter? I have two different spot meters, a Pentax and a Soligor. The Pentax reads 2/3 stop faster than the Soligor at low levels and 1/2 stop faster at high levels. My Weston and various camera meters all give different readings from the same subject under the same light, but vary differently at different light levels. Similarly, the llford digital thermometer I use is 1'C adrift from my Kodak mercury version. When all the other possible variables in taking and processing, such as shutter speeds, real lens light transmission compared to marked aperture, flare factors, lens extension, agitation technique etc, are taken into account it's not surprising that practice deviates from laboratory theory.

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